Method for monitoring and controlling combustion in fuel gas burner apparatus, and combustion control system operating in accordance with said method

ABSTRACT

A method is provided for monitoring and controlling combustion in a burner of a fuel gas apparatus, having a sensor with an electrode able to be supplied by a voltage generator and connected to an electronic circuit for measuring the resultant potential. The method includes acquiring and processing data from experimental conditions and a second phase of evaluating the desired combustion characteristic, under an actual operating condition of the burner. A plurality of experimental combustion conditions for the burner are preselected, applying to the burner, in each condition, a power and a further significant parameter of the combustion characteristics, under each of the experimental conditions applying an electrical voltage signal to said electrode and carrying out a sampling of the response signal, calculating, based on the sequence of sampled values, the characteristic parameters of the waveform of the signal for each of the experimental conditions.

TECHNICAL CONTEXT

The present invention relates to a method for monitoring and controllingcombustion in fuel gas burners for apparatus such as boilers, hot watercylinders, fireplaces and the like, with the features mentioned in thepreamble of the main claim. It also relates to a combustion controlsystem operating in accordance with said method.

TECHNOLOGICAL BACKGROUND

In the reference technical sector it is known that, to maintainefficient combustion, it is necessary for the ratio between the amountof air and the amount of fuel gas introduced into the burner to bemaintained at around a predetermined optimal value, which dependssubstantially on the type of gas used and, in general, can also dependon the value of the power delivered by the burner, i.e. by the gas flowrate.

In this way a complete combustion process can be achieved and maintainedover time without excessive energy loss as fumes, while minimising theproduction of polluting gases and complying with emissions legislationin the various countries.

To achieve this objective of maintaining the optimal air/gas ratio,various devices and methods have been developed in the referencetechnical sector.

In the specific scope of the invention, there are known methods formonitoring and controlling combustion on the basis of flame analysisand, in particular, analysis of the gas ionisation in the combustionzone of the flame. Typical methods provide for the use of an electrodewhich is placed in or close to the flame zone and connected to anelectronic circuit that applies a fixed or variable voltage to theelectrode and measures the current passing through said electrode. Oneor more combustion-related parameters are estimated by means of systemsfor processing and analysing the current signal. The processing systemsinclude known methods for analysing the frequency spectrum of thesignal, which analysis is capable of identifying frequency spectra orvariations of the same that indicate flame instability or sub-optimalcombustion, on the basis of which, systems for correcting the combustionare provided in order to return the latter to the desired conditions.

Identifiable limitations of the known methods relate mainly to thereliability of the results of the frequency spectrum analyses and totheir correlation with the combustion process.

Limitations can also be encountered in the possible wear and ageing ofthe electrode for receiving the signal in the ionisation sensor, withconsequent repercussions on the reliability and accuracy of the dataanalysed by the frequency spectrum processing algorithms.

The aforesaid limitations are also amplified if the combustion controlis to be carried out in burners of the modulating type, in which optimalcombustion conditions are sought by varying the required power, withinthe range between minimum power and maximum admissible power for theburner.

It is also known that the volumetric ratio between the gas flow rate andthe air flow rate appropriate for correct combustion also depends on thetype of gas. Therefore, each family of fuel gases is correlated withrespective, specific control curves (which, for example, correlate thegas flow rate with the air flow rate). One of the problems of knownsystems for controlling combustion consists is identifying the family ofgases and associating the optimal control curves.

DESCRIPTION OF THE INVENTION

The problem addressed by the present invention is that of producing amethod for monitoring and controlling combustion in a burner of fuel gasapparatus, and also a combustion control system operating in accordancewith said method, which are structurally and functionally designed toovercome the limitations set out above with reference to the cited priorart.

Within the context of this problem, one object of the invention is tomake available a control method and system that are capable of ensuringoptimal combustion throughout the range of flow rates (and for variousgas types), i.e. the powers for which the burner size is intended,ensuring reliable and repeatable results when analysing signalscorrelated with the combustion process.

Another object of the invention is to offer a control method and systemthat is simple to manage and characterise, during both installation anduse of the burner of the apparatus.

This problem is solved and these objects are achieved by the presentinvention through a method and a system for controlling combustion in aburner of a fuel gas apparatus, implemented according to the claimswhich follow.

BRIEF DESCRIPTION OF THE DRAWINGS

The features and advantages of the invention will become more apparentfrom the detailed description of a preferred embodiment thereof, shownnon-restrictively and for information with reference to the attacheddrawings in which:

FIG. 1 is a diagrammatic view of a burner of an apparatus provided witha combustion control system operating according to the method formonitoring and controlling combustion according to the invention,

FIG. 2 is a graph showing the curves of correlation between operatingparameters of a fan and of a modulating gas valve of a burner apparatusimplementing the combustion control method of the invention.

PREFERRED EMBODIMENTS OF THE INVENTION

Referring first to FIG. 1, the numeral 1 indicates overall a burner thatis provided with a combustion control system, produced so as to operateaccording to the method for monitoring and controlling combustion of thepresent invention.

The burner 1 is housed in an apparatus (not shown) intended for theproduction of domestic hot water and/or coupled to a space-heatingsystem, in a manner known per se and not shown in the drawings.

The burner 1 comprises a combustion chamber 2, which is supplied by afirst 3 and a second 4 duct, configured so as to introduce into thecombustion chamber 2 a flow of air and, respectively, a flow of fuelgas. Preferably, the second duct 4 enters the first duct 3 upstream ofthe combustion chamber 2 (premixing burner). In the air-gas mixingsection, a fan 5 is provided, with a variable rotation speed. Thenumeral 6 indicates a modulating valve placed on the gas duct 4 tocontrol the flow rate of gas introduced into the burner.

The combustion chamber 2 is connected downstream to a chimney 7, throughwhich the exhaust gases from combustion are discharged.

The numeral 8 indicates a combustion monitoring sensor, described ingreater detail below, which is connected to a control device 9 providedwith an electronic circuit suitable for controlling the burner accordingto the method of the present invention, as shown below. The controldevice is further connected operationally both to the fan 5 and themodulating valve 6, so as to control those members.

The sensor 8 is positioned close to the burner flame, the burner beingcapable of receiving a supply from a voltage generator and is also beingconnected to an electronic circuit suitable for measuring the resultantpotential at the sensor.

One embodiment provides for the sensor 8 to comprise two electrodes,indicated as E1, E2, which are placed inside or close to the flame. Asan alternative, provision is made for the use of a single electrode, towhich the voltage signal is applied and, following the disconnection ofsaid signal, the response signal is immediately acquired by means of aseries of samplings of the latter.

From what is known from physics about the plasmas that develop incombustion processes, if a charge is introduced into the plasma fromoutside, the electrical field produced by said charge results in motionof the charges constituting the plasma; this motion increases in linewith the increase in the introduced external charge. However, there isan electrical field value beyond which the flow of charged particlesincreases no further (saturation). The motion varies considerably interms of electrons and ions: the electrons, being much lighter andsmaller, move much faster and suffer far fewer collisions along theirpath. This means that the aforesaid saturation phenomenon arises muchearlier in the case of positive ions, while it happens later forelectrons. Owing to the displacement of charged particles, themacroscopic effect generated by the introduced external charge is achange in the electrical field of the plasma. This electrical field ispropagated around the particle by a distance of the order of the “Debyelength”. In connection with the above, this is greater for electrons,i.e. where the introduced charge is positive. In contrast, it will bemuch smaller for positive ions, corresponding to the case where theintroduced charge is negative.

Returning to the method of the invention, an electrical signal having agiven waveform over time is applied to the electrode E1; this potentialis equivalent to the perturbing charge mentioned earlier in thedescription. The electrode E2 is located at a suitable distance andtakes a value for potential determined by the motion of the plasmacharges caused by E1 and responding to the dynamics described above.This potential is measured by the electronic circuit and processed asdescribed below.

The basic concept of the method of the invention is therefore that theresultant waveform at the electrode E2 is determined unambiguously bythe composition of the mixture of oxidising agent and fuel beforecombustion. It is essential to know this composition in order to be ableto predict any key effects of combustion, such as the amount of CO₂ andCO produced and the thermal power produced. In this way, it is possibleamong other things to compensate for the effects of gases other than thenominal ones, indicated in the sector as G20 and G31. Therefore, if weknow the air/fuel ratio (air number otherwise marked as “λ”), it ispossible to produce a combustion control system for a gas burnerapparatus.

The method of the invention essentially comprises two macro operatingphases, a first phase, referred to as F, of acquiring and processingdata from experimental conditions, and a second phase, referred to as H,aimed at evaluating the air number λ or the amount of CO₂ and COproduced or the thermal power produced, under an actual operatingcondition of the burner.

In turn, both of these phases comprise a sequence of operating steps,which are described in detail below.

The following description sets out the steps relating to evaluation ofthe air number λ, but they can be applied in the same way for otherparameters correlated with combustion. Below, this significant parameterof the characteristics of combustion will also be referred to, in moregeneral terms, as K and this, in addition to the power P of the burner,can be selected, for example, as the air number λ or as theconcentration (% or ppm) of CO₂ or CO emitted in the combustion process,it being understood that further significant parameters of combustioncan also be preselected, as an alternative.

A first operating step of phase F, shown as F1, provides for identifyinga plurality (1, 2, . . . , n) of experimental combustion conditions ofthe burner, in each of which a respective power P (P1, P2, . . . , Pn)is set at a number n of levels and for each power an air number value(λ1, λ2, . . . , λm) is set, selected at a number m of levels, the airnumber λ expressing the ratio between the amount of air in thecombustion process and the amount of air for stoichiometric combustion,each power level n being associated with the respective levels m of theair number, each experimental condition further being repeated apredetermined number r of times. In other words, a grid (m*n) of pairsof values P, λ is produced, in which for each pair of values thecondition is repeated r times.

As an alternative, in each experimental condition a power P (P1, P2, . .. , Pn) can be set and for each power a concentration of CO₂ and/or CO(% 1, % 2, . . . % n) is set. In this case too, each experimentalcondition is repeated a predetermined number of times (r).

A second, successive operating step, shown as F2, provides for anelectrical signal to be applied to the electrode E1 in each of said(n*m*r) experimental conditions (Pi, λj or Pi, % j).

Reference will be made below to the selection of experimental conditionswith the power and air number being set, it being understood that themethod can be applied analogously in the alternative selection ofexperimental conditions with the power and CO₂ (and/or CO) concentrationbeing set.

In a third step F3 the resultant signal at the electrode E2 is sampled,calculating the respective characteristic parameters of the waveform ofthe signal for each of the aforesaid experimental conditions. The term“sampling” means, in greater detail, a series of samplings of theresponse signal measured at the electrode, in which an analogue/digitalconversion of the voltage measured at the electrode is obtained atregular intervals and for a defined duration.

A further, subsequent operating step, shown as F4, provides forcalculating a correlation function, on the basis of the acquiredexperimental data, capable of unambiguously correlating the power P, theair number λ and the characteristic parameters of the waveform of thesignal at the electrode E2, in the combustion process of the burner.

The characteristic parameters of the waveform are advantageouslyobtained by means of techniques of harmonic analysis of the voltagesignal sampled by application of a functional transform. Examples ofpossible choices of functional transform are the Hartley transform orthe Fourier transform.

Moreover, the correlation function, which allows the characteristicparameters of the measured waveform to be correlated with the air numberλ and the power P, is obtained by application of regression analysistechniques.

In other words, the mechanism allowing the waveform measured at theelectrode E2 to be correlated with the air number λ is of the “patternmatching” type and is implemented by applying regression analysistechniques.

In one embodiment, in phase F2, a voltage signal with a periodicwaveform, such as a sinusoidal waveform, is applied to the electrode E1at a constant amplitude M and a given frequency f.

In a preferred embodiment, use is made of a single electrode E1, and theaforesaid operating steps F2 and F3 are performed in immediatesuccession on the same single electrode. In other words, the electricalvoltage signal is applied to the electrode and, following thedisconnection of the signal applied, a series of samplings of theresultant response signal at the electrode is carried out. The discreteFourier transform (DFT) is applied to the waveform of the signal sampledat the electrode E2, at the frequency of the waveform of the electrodeE1 and at its subsequent harmonics, obtaining the amplitude M and phaseφ for said frequencies.

This operation is carried out for each of the aforesaid experimentalconditions, corresponding to the preselected powers (P1, P2, . . . ,Pn), and for each of these at the air number values (λ1, λ2, . . . ,λm), carrying out a predetermined number (r) of repetitions for each ofsaid conditions, for a total number of observations equal to n*m*r.

At this point, provision is made for:

-   -   calculating, for each experimental condition (i, j), the        amplitudes (M1 i,j, M1 i,j, . . . Mpi,j) and phases (φ1 i,j, φ2        i,j, . . . , φpi,j) by applying the discrete Fourier transform        (DFT), where p is the harmonic maximum for which the discrete        Fourier transform (DFT) is applied,    -   inserting the amplitude (M) and phase (φ) values into a linear        system in which each row is composed of an experimental        observation made at the power Pi and at the air number λj and in        which the known term is λj,    -   setting a number of experimental observations (n*m*r) which is        greater than the maximum number of harmonics (p), at least equal        to 3p-2,    -   solving the linear system of the equation AB=λ        with A being the matrix of experimental data, B the vector of        the unknown coefficients and λ the vector, by the least-squares        regression method, of the Moore-Penrose equation where

B=(ATA)⁻¹ A ^(T)

-   -   storing in the electronic circuit the coefficient vector B, with        a dimension equal to the unknowns of the system or equal to the        number of columns of the matrix A, so as to use the following        regression equation:

${\lambda_{j} = \left\lbrack {1\mspace{14mu} \left( \frac{M_{2}}{M_{1}} \right)^{s}\mspace{11mu} \left( \frac{M_{3}}{M_{1}} \right)^{s}\mspace{11mu} \left( \frac{M_{4}}{M_{1}} \right)^{s}\mspace{11mu} \left( \frac{M_{5}}{M_{1}} \right)^{s}\mspace{11mu} \ldots \mspace{11mu} \left( \frac{M_{p}}{M_{1}} \right)^{s}\mspace{11mu} {\sin \left( {\phi_{2} - {2r\; \phi_{1}}} \right)}\; {\sin \left( {\phi_{3} - {3\; r\; \phi_{1}}} \right)}{\sin \left( {\phi_{4} - {4\; r\; \phi_{1}}} \right)}{\sin \left( {\phi_{5} - {5\; r\; \phi_{1}}} \right)}\mspace{11mu} \ldots \mspace{11mu} {\sin \left( {\phi_{p} - {p\; r\; \phi_{1}}} \right)}\mspace{11mu} {\cos \left( {\phi_{2} - {2\; r\; \phi_{1}}} \right)}\mspace{14mu} {\cos \left( {\phi_{3} - {3\; r\; \phi_{1}}} \right)}\; {\cos \left( {\phi_{4} - {4\; r\; \phi_{1}}} \right)}{\cos \left( {\phi_{5} - {5\; r\; \phi_{1}}} \right)}\mspace{11mu} \ldots \mspace{11mu} {\cos \left( {\phi_{p} - {p\; r\; \phi_{1}}} \right)}} \right\rbrack}\;$

with s and r which may assume a value in the range [1;4] and p≧5.

Preferred values of p are between 5 and 15.

In phase H of the method, relating to an operating condition of actualfunctioning of the burner, the following operating steps are provided,to evaluate the air number λ.

A first operating step, referred to as H1, provides for applying thevoltage signal to the electrode E1.

Simultaneously (in step H2) provision is made for acquiring theelectrical signal at the second electrode (E2) for a predetermined timeinterval, as described in phase F2.

In a preferred embodiment, use is made of a single electrode E1, and theaforesaid operating steps H1 and H2 are performed in immediatesuccession on the same single electrode.

In a third, successive step H3, the amplitude (M1, M2, . . . , Mp) andphase (φ1, φ2, . . . , φp) of the waveform of the resultant voltagesignal at the electrode E2 are calculated by means of discrete Fouriertransform, while in a fourth step H4 the estimated air number value(λstim) is calculated by means of the following scalar product:

$\lambda_{stim} = {\left\lbrack {1\mspace{14mu} \left( \frac{M_{2}}{M_{1}} \right)^{s}\mspace{11mu} \left( \frac{M_{3}}{M_{1}} \right)^{s}\mspace{11mu} \left( \frac{M_{4}}{M_{1}} \right)^{s}\mspace{11mu} \left( \frac{M_{5}}{M_{1}} \right)^{s}\mspace{11mu} \ldots \mspace{11mu} \left( \frac{M_{p}}{M_{1}} \right)^{s}\mspace{11mu} {\sin \left( {\phi_{2} - {2r\; \phi_{1}}} \right)}\; {\sin \left( {\phi_{3} - {3\; r\; \phi_{1}}} \right)}{\sin \left( {\phi_{4} - {4\; r\; \phi_{1}}} \right)}{\sin \left( {\phi_{5} - {5\; r\; \phi_{1}}} \right)}\mspace{11mu} \ldots \mspace{11mu} {\sin \left( {\phi_{p} - {p\; r\; \phi_{1}}} \right)}\mspace{11mu} {\cos \left( {\phi_{2} - {2\; r\; \phi_{1}}} \right)}\mspace{11mu} {\cos \left( {\phi_{3} - {3\; r\; \phi_{1}}} \right)}\; {\cos \left( {\phi_{4} - {4\; r\; \phi_{1}}} \right)}{\cos \left( {\phi_{5} - {5\; r\; \phi_{1}}} \right)}\mspace{11mu} \ldots \mspace{11mu} {\cos \left( {\phi_{p} - {p\; r\; \phi_{1}}} \right)}} \right\rbrack \times B}$

using the correlation function, which correlates the power and the airnumber λ with the characteristic parameters of the waveform observed.

λ can be calculated at predetermined regular intervals, as will beexplained in detail below.

Preferably, in the phase of harmonic analysis of the waveform of thesignal associated with the electrode E2, provision is made forcalculating the amplitude and phase of a preselected number ofharmonics.

Advantageously, provision can be made for calculating, in said firstphase F of the method, a plurality of vectors B of calibrationcoefficients, each correlated with respective power bands between theminimum and maximum admissible power, which bands overlap at least inpart, in order to achieve greater precision in estimating the airnumber. For example, three distinct vectors Blow, Bmed and Bhi can beused respectively in three partially superimposed power bands: low,medium and high power. In this way, greater accuracy is obtained than byusing a single vector B. Each vector has been determined by using thepowers referring to it.

Provision can also be made for calculating a coefficient vector Bfamcorrelated with the respective gas family for which the burner isintended, so as to allow said gas family to be identified during theburner installation phase. Using Bfam it is possible to estimate the airnumber independently of the family to which the gas belongs. It is lessaccurate than other vectors B and can be used only for identifying thefamily in the installation phase of the apparatus. This simplifies theprocedure of installing the burner.

Alternatively, using a method of the aforesaid type, the power can alsobe estimated, and this may be different from that normally estimated inan open loop, for example by using gases other than the reference gasfor the family or for the purposes of adjusting the device formodulating the gas flow rate or for the characteristics of theinstallation (for example of the application type, relating to thelength of the fume discharge duct or if it becomes blocked). Thisestimated power value can be used in the aforesaid combustion controlsystem, to adjust power also in a closed loop. In this way it ispossible also to simplify the procedure for installing the apparatus,with a consequent time-saving.

By using the aforesaid method it is also possible to diagnose conditionsof the apparatus that differ from the nominal ones, for exampledetermined by an out-of-tolerance positioning of the electrode or causedby deterioration of the electrode through ageing. All that is needed todo this is to use, instead of λj, a suitable parameter representing thecondition of the apparatus (nominal or anomalous) prevailing in theexperiment j.

Periodic voltage signals can also be applied to the electrode E1, not ata single frequency but at several frequencies in succession, so thateach frequency excites the specific characteristics of the plasma.Alternatively it is possible to apply certain frequencies for certainpower levels and other frequencies for other power levels.

It is also possible to apply to E1 a waveform constituted by asuperimposed sinusoid at a constant level with a greater value. In thatcase the parameters observable at E2 are the modulus and phase of thesinusoid of the same frequency and its harmonics and the mean value.

A principal variant of the method of the invention provides for thesensor 8 to be of the single-electrode type, in which the singleelectrode E1 is supplied with a preselected electrical signal.Preferably, the electrode E1 is supplied with a periodic, pulsed voltagesignal.

In a first configuration, the voltage signal comprises, over the signalperiod, a first pulse with a positive amplitude followed by a secondpulse with a negative amplitude. As an alternative, the voltage signalcomprises, over the period, a pulse with a positive or negativeamplitude.

Advantageously, the frequency of the pulsed signal at the electrode E1is a function of the power delivered to the burner and, additionally,the sampling frequency is a function of the power delivered to theburner.

Provision can be made for a first sampling frequency of the signalassociated with the first pulse and a second, distinct samplingfrequency associated with the second pulse.

By analogy with the methods using a dual-electrode sensor, the method inthe variant with a single-electrode sensor also provides for:

-   -   applying to the waveform observed at the electrode E1 a        functional transform, for example the discrete Fourier transform        (DFT) at the preselected frequency and at its subsequent        harmonics, obtaining the amplitude (M) and phase (D) for said        frequencies,    -   carrying out said operation for each of said experimental        conditions corresponding to the powers (P1, P2, . . . , Pn), and        for each of these at the air number values (λ1, λ2, . . . , λm),        carrying out a predetermined number (r) of repetitions for each        of said conditions, for a total number of observations equal to        n*m*r,    -   calculating, for each experimental condition (i, j), the        amplitudes (M1 i,j, M2 i,j, . . . Mpi,j) and phases (φ1 i,j, φ2        i,j, . . . , φpi,j) by applying the discrete Fourier transform        (DFT),        where p is the harmonic maximum for which the discrete Fourier        transform (DFT) is applied,    -   inserting the amplitude (M) and phase (φ) values into a linear        system in which each row is obtained from an experimental        observation made at the power Pi and at the air number λj and in        which the known term is λj,    -   setting a number of experimental observations (n*m*r) which is        greater than the maximum number of harmonics (p),    -   solving the linear system of the equation AB=λ        with A being the matrix of experimental data, B the vector of        the unknown coefficients and λ the vector, by the least-squares        regression method, of the Moore-Penrose equation where

B=(A ^(T) A)⁻¹ A ^(T)

-   -   storing in the electronic circuit the coefficient vector B, with        a dimension equal to the unknowns of the system or equal to the        number of columns of the matrix A, so as to use the following        regression equation:

${\lambda_{j} = \left\lbrack {1\mspace{14mu} \left( \frac{M_{2}}{M_{1}} \right)^{s}\mspace{11mu} \left( \frac{M_{3}}{M_{1}} \right)^{s}\mspace{11mu} \left( \frac{M_{4}}{M_{1}} \right)^{s}\mspace{11mu} \left( \frac{M_{5}}{M_{1}} \right)^{s}\mspace{11mu} \ldots \mspace{11mu} \left( \frac{M_{p}}{M_{1}} \right)^{s}\mspace{11mu} {\sin \left( {\phi_{2} - {2r\; \phi_{1}}} \right)}\; {\sin \left( {\phi_{3} - {3\; r\; \phi_{1}}} \right)}{\sin \left( {\phi_{4} - {4\; r\; \phi_{1}}} \right)}{\sin \left( {\phi_{5} - {5\; r\; \phi_{1}}} \right)}\mspace{11mu} \ldots \mspace{11mu} {\sin \left( {\phi_{p} - {p\; r\; \phi_{1}}} \right)}\mspace{11mu} {\cos \left( {\phi_{2} - {2\; r\; \phi_{1}}} \right)}\mspace{14mu} {\cos \left( {\phi_{3} - {3\; r\; \phi_{1}}} \right)}\; {\cos \left( {\phi_{4} - {4\; r\; \phi_{1}}} \right)}{\cos \left( {\phi_{5} - {5\; r\; \phi_{1}}} \right)}\mspace{11mu} \ldots \mspace{11mu} {\cos \left( {\phi_{p} - {p\; r\; \phi_{1}}} \right)}} \right\rbrack}\;$

In this variant too, in phase H of the method, relating to an operatingcondition of actual functioning of the burner, the following operatingsteps are provided, to evaluate the air number λ.

A first step H1 provides for acquiring the voltage signal at theelectrode E1 for a predetermined time interval; in a second, successivestep H2, the amplitude (M1, M2, . . . , Mp) and phase (φ1, φ2, . . . ,φp) of the waveform of the signal acquired at the electrode E2 arecalculated by means of discrete Fourier transform, while in a third stepH3 the estimated air number value (λstim) is calculated by means of thefollowing scalar product:

$\lambda_{stim} = {\left\lbrack {1\mspace{14mu} \left( \frac{M_{2}}{M_{1}} \right)^{s}\mspace{11mu} \left( \frac{M_{3}}{M_{1}} \right)^{s}\mspace{11mu} \left( \frac{M_{4}}{M_{1}} \right)^{s}\mspace{11mu} \left( \frac{M_{5}}{M_{1}} \right)^{s}\mspace{11mu} \ldots \mspace{11mu} \left( \frac{M_{p}}{M_{1}} \right)^{s}\mspace{11mu} {\sin \left( {\phi_{2} - {2r\; \phi_{1}}} \right)}\; {\sin \left( {\phi_{3} - {3\; r\; \phi_{1}}} \right)}{\sin \left( {\phi_{4} - {4\; r\; \phi_{1}}} \right)}{\sin \left( {\phi_{5} - {5\; r\; \phi_{1}}} \right)}\mspace{11mu} \ldots \mspace{11mu} {\sin \left( {\phi_{p} - {p\; r\; \phi_{1}}} \right)}\mspace{11mu} {\cos \left( {\phi_{2} - {2\; r\; \phi_{1}}} \right)}\mspace{11mu} {\cos \left( {\phi_{3} - {3\; r\; \phi_{1}}} \right)}\; {\cos \left( {\phi_{4} - {4\; r\; \phi_{1}}} \right)}{\cos \left( {\phi_{5} - {5\; r\; \phi_{1}}} \right)}\mspace{11mu} \ldots \mspace{11mu} {\cos \left( {\phi_{p} - {p\; r\; \phi_{1}}} \right)}} \right\rbrack \times B}$

using the correlation function, which correlates the power and the airnumber λ with the characteristic parameters of the waveform observed.

λ can be calculated at predetermined regular intervals, as will beexplained in detail below.

To summarise the preceding phases it can therefore be stated that theparameters of the mathematical model relating to the correlationfunction, in combination with the functional transform of the waveformsacquired following the stimulus applied to the plasma, are capable ofcalculating the desired combustion characteristics.

It should be noted that, unlike known methods for monitoring andcontrolling combustion, the method of the invention is based onmeasuring voltage rather than on measuring the ionisation current, andis therefore less subject to problems arising from wear and ageing ofthe electrodes.

Moreover, to determine the calibration parameters (vector B), apredetermined, relatively limited number of experimental tests isrequired, thus permitting shorter fine-tuning times than in the priorart.

A combustion control and adjustment system for the burner 1, operatingby the method of the invention, provides for example for the followingoperating phases, with reference to the graph in FIG. 2, where thex-axis shows the number of rotations (n) of the fan, the y-axis in itsupper quadrant expressing the current (I) for actuating the modulatinggas valve, the y-axis in its lower quadrant expressing the flow rate (Q)of gas delivered (correlated with the power requirements).

The adjustment curves c of the aforesaid parameters are typically presetin the control circuit, as shown in the diagram. Therefore, for example,a requirement Q1 has a corresponding number of rotations n1 and currentI1.

If the power requirement changes from Q1 to Q2, the number of rotationsrises to n2, in which condition the control circuit associates thecurrent value I2 with the modulator. Said values are correlated with atarget air number (λob) that is deemed optimal for combustion. In thisnew operating condition, therefore, the effective air number (λstim) isestimated using the method described above and a comparison is madebetween λob and λstim, making the appropriate corrections to theparameters—current I—or—number of rotations n—to arrive at an air numberwhich basically coincides with the target air number. Preferably, thecurrent at the modulator is varied, for example raised to the value I2′.At this point the operating curve c is updated again, for the air numberequal to the target air number, which then becomes the curve c′.

The control curve can, for example, be updated by accumulating a certainnumber of correction points and calculating the regression curvecorrelating said points, this curve becoming the new control curve.Alternatively, it is possible only to make a correction, whereappropriate, at each operating point, on the basis of the comparisonλob/λstim—without identifying a new operating curve (by means of linearregression).

The adjustment system described above simply represents a non-exhaustiveexample, for the purposes of applying the method of combustionmonitoring and control of the invention. It will be understood that thismethod makes it possible to provide specific principles for controllingand adjusting the burner operation, according to the respectiveoperating and system requirements, which in any case provide for thecomparison between a target air number that is optimal for combustionand the air number estimated by the method of the invention.

The invention therefore achieves the proposed aims, overcoming thelimitations revealed in the prior art and demonstrating the advantagesover known solutions, as stated.

It should be noted that the method of the invention provides for theacquisition of waveforms which are variable over time, this aspectconstituting a feature that, together with the logic for data processingand computing, has a decisive effect on the accuracy and stability ofthe method and of the control system according to the invention. Such aproperty differs substantially from the known solutions in whichreference is made to currents measured in stationary mode or tostationary measurements of significant parameters of combustion.

It will also be observed that the method of the invention provides forperturbation to be applied to the plasma of the flame (voltage signalapplied to the electrode) and, subsequently, once the signal isdisconnected, the response signal is acquired from the voltage meter. Inthis manner, stimulus and measurement occur in two distinct, separatephases. This aspect differs substantially from the known solutions, inwhich the voltage signal is applied and the effects are observed at thesame time, resulting in a mingling of stimulus and response that makesit harder to distinguish one from the other and makes the measurementintrusive and subject to the characteristics of the stimulus, i.e. theelectrode and its state of wear and oxidisation.

Furthermore, based on the acquisition of time-domain waveforms, themethod of the invention makes it possible to process richer and morecomplete information on the state of combustion; in fact, what isobserved is the dynamic response of the plasma to the stimulus given,rather than the mean response in stationary conditions.

It should also be noted that the model obtained with the method of theinvention is valid throughout the operating range of the system, both indesired and undesired operating conditions. It follows that noadditional models are needed in order to recognise extreme conditions,for example those involving excessive emission of noxious gases or noisyoperation.

1. Method for monitoring and controlling combustion in a burner (1) of afuel gas apparatus, of the type comprising a sensor (8) with anelectrode (E1) located in or close to a flame and configured to besupplied by a voltage generator and also connected to an electroniccircuit suitable for measuring the resultant potential at the electrode(E1), the method comprising: a first phase of acquiring and processingdata from experimental conditions comprising the following steps:identifying a plurality of experimental combustion conditions for theburner (1), for each of said conditions applying to the burner arespective power (P1, P2, . . . , Pn) of a number n of preselected powerlevels and a further significant parameter of the combustioncharacteristics (K1, K2, . . . , Km), at a number m of levels,associating with each level n of power the respective levels m of saidfurther parameter, each experimental condition being repeated apredetermined number r of times, applying, in each of said (n*m*r)experimental conditions, an electrical voltage signal to said electrode(E1) and, after disconnecting the signal applied to the electrode,carrying out a series of samplings of the resultant response signal atthe electrode, calculating, on the basis of the sequence of sampledvalues, the respective characteristic parameters of the waveform of saidresponse signal for each of said experimental conditions, calculating acorrelation function on the basis of the acquired experimental data,capable of unambiguously correlating said power (P) and said furthersignificant parameter (K) of the combustion characteristics with thecharacteristic parameters of the waveform of the signal at the electrode(E1), in the combustion process of the burner (1), and a second phase ofevaluating the significant parameters of the combustion characteristics,under an actual operating condition of the burner (1), comprising thefollowing steps: applying, under said actual operating condition, anelectrical voltage signal to said electrode (E1) and, following thedisconnection of the signal applied to the electrode, carrying out aseries of samplings of the resultant response signal at the electrode,calculating, on the basis of the sequence of sampled values, therespective characteristic parameters of the waveform of said responsesignal for said operating condition, calculating the estimated value ofthe desired combustion characteristic by using said correlationfunction.
 2. The method according to claim 1, wherein said furthersignificant parameter of the combustion characteristics is chosen atleast from the air number (λ), understood as the ratio between theamount of air in the combustion process and the amount of air forstoichiometric combustion, and the CO₂ or CO concentration in thecombustion process.
 3. The method according to claim 1, wherein thecharacteristic parameters of the waveform of the response signals areobtained by applying a functional transform.
 4. The method according toclaim 1, wherein the correlation function, which allows the measuredwaveform to be correlated with the significant parameter of thecombustion characteristics, is obtained by application of regressionanalysis techniques.
 5. The method according to claim 1, wherein aperiodic, pulsed voltage signal is applied to the electrode (E1).
 6. Themethod according to claim 1, wherein said pulsed voltage signalcomprises, over the signal period, a first pulse with a positiveamplitude followed by a second pulse with a negative amplitude.
 7. Themethod according to claim 1, wherein said pulsed voltage signalcomprises, over the signal period, a pulse with a positive or negativeamplitude.
 8. The method according to claim 2 further comprising:applying to the electrode (E1) a voltage with a pulsed, alternatingwaveform at a constant amplitude (M) and with a predetermined frequency(f), acquiring the response signal after each individual pulse at theelectrode, applying to the waveform of the signal acquired at theelectrode a discrete Fourier transform (DFT) at the frequency of thewaveform of the electrode and at its subsequent harmonics, obtaining theamplitude (M) and phase (Φ) for said frequencies, carrying out saidoperation for each of said experimental conditions, corresponding to thepowers (P1, P2, . . . , Pn), and for each of these at the air numbervalues (λ1, λ2, . . . , λm), carrying out a predetermined number (r) ofrepetitions for each of said conditions, with a total number ofobservations equal to n*m*r, calculating, for each experimentalcondition (i, j), the amplitudes (M1 i,j, M2 i,j, . . . Mpi,j) andphases (Φ1 i,j, Φ2 i,j, . . . , ΦPpi,j) by applying the discrete Fouriertransform (DFT), where p is the harmonic maximum for which the discreteFourier transform (DFT) is applied, inserting the amplitude (M) andphase (Φ) values into a linear system in which each row is obtained froman experimental observation made at the power Pi and the air number λjand in which the known term is λj, setting a number of experimentalobservations (n*m*r) which is greater than the maximum number ofharmonics (p), at least equal to 3p-2 solving the linear system of theequation AB=λ with A being the matrix of experimental data, B the vectorof the unknown coefficients and λ the vector, by the least-squaresregression method, of the Moore-Penrose equation whereB=(A ^(T) A)⁻¹ A ^(T) storing in the electronic circuit the coefficientvector B, with a dimension equal to the unknowns of the system or equalto the number of columns of the matrix A, so as to use the followingregression equation:${\lambda_{j} = \left\lbrack {1\mspace{14mu} \left( \frac{M_{2}}{M_{1}} \right)^{s}\mspace{11mu} \left( \frac{M_{3}}{M_{1}} \right)^{s}\mspace{11mu} \left( \frac{M_{4}}{M_{1}} \right)^{s}\mspace{11mu} \left( \frac{M_{5}}{M_{1}} \right)^{s}\mspace{11mu} \ldots \mspace{11mu} \left( \frac{M_{p}}{M_{1}} \right)^{s}\mspace{11mu} {\sin \left( {\phi_{2} - {2r\; \phi_{1}}} \right)}\; {\sin \left( {\phi_{3} - {3\; r\; \phi_{1}}} \right)}{\sin \left( {\phi_{4} - {4\; r\; \phi_{1}}} \right)}{\sin \left( {\phi_{5} - {5\; r\; \phi_{1}}} \right)}\mspace{11mu} \ldots \mspace{11mu} {\sin \left( {\phi_{p} - {p\; r\; \phi_{1}}} \right)}\mspace{11mu} {\cos \left( {\phi_{2} - {2\; r\; \phi_{1}}} \right)}\mspace{14mu} {\cos \left( {\phi_{3} - {3\; r\; \phi_{1}}} \right)}\; {\cos \left( {\phi_{4} - {4\; r\; \phi_{1}}} \right)}{\cos \left( {\phi_{5} - {5\; r\; \phi_{1}}} \right)}\mspace{11mu} \ldots \mspace{11mu} {\cos \left( {\phi_{p} - {p\; r\; \phi_{1}}} \right)}} \right\rbrack}\;$ with s and r which may assume a value in the range [1;4] and p≧5,estimating the air number value, under an actual operating condition, bymeans of the following steps: acquiring the voltage signal at theelectrode for a predetermined time interval, calculating the amplitude(M1, M2, . . . , Mp) and phase (Φ1, Φ2, . . . , Φp) by means of discreteFourier transform, and calculating the estimated air number value(λstim) by the following scalar product:$\lambda_{stim} = {\left\lbrack {1\mspace{14mu} \left( \frac{M_{2}}{M_{1}} \right)^{s}\mspace{11mu} \left( \frac{M_{3}}{M_{1}} \right)^{s}\mspace{11mu} \left( \frac{M_{4}}{M_{1}} \right)^{s}\mspace{11mu} \left( \frac{M_{5}}{M_{1}} \right)^{s}\mspace{11mu} \ldots \mspace{11mu} \left( \frac{M_{p}}{M_{1}} \right)^{s}\mspace{11mu} {\sin \left( {\phi_{2} - {2r\; \phi_{1}}} \right)}\; {\sin \left( {\phi_{3} - {3\; r\; \phi_{1}}} \right)}{\sin \left( {\phi_{4} - {4\; r\; \phi_{1}}} \right)}{\sin \left( {\phi_{5} - {5\; r\; \phi_{1}}} \right)}\mspace{11mu} \ldots \mspace{11mu} {\sin \left( {\phi_{p} - {p\; r\; \phi_{1}}} \right)}\mspace{11mu} {\cos \left( {\phi_{2} - {2\; r\; \phi_{1}}} \right)}\mspace{11mu} {\cos \left( {\phi_{3} - {3\; r\; \phi_{1}}} \right)}\; {\cos \left( {\phi_{4} - {4\; r\; \phi_{1}}} \right)}{\cos \left( {\phi_{5} - {5\; r\; \phi_{1}}} \right)}\mspace{11mu} \ldots \mspace{11mu} {\cos \left( {\phi_{p} - {p\; r\; \phi_{1}}} \right)}} \right\rbrack \times B}$9. The method according to claim 8, wherein the sampling frequency is afunction of the power delivered to the burner (1).
 10. The methodaccording to claim 8, wherein there is a first sampling frequency of thesignal associated with the positive pulses and a second, distinctsampling frequency associated with the negative pulses.
 11. The methodaccording to claim 1, wherein the sensor provided is of the dualelectrode type with a first and a second electrode (E1, E2), arranged ata predetermined mutual spacing, a voltage having a specific waveformover time being applied to the first electrode, the potential assumed bythe second electrode being measured and processed by the electroniccircuit, by means of said sampling and harmonic analysis of therespective waveform.
 12. The method according to claim 11, furthercomprising: applying to the first electrode (E1) a voltage with aperiodic waveform at a constant amplitude (M) and with a predeterminedfrequency (f), applying to the waveform observed at the second electrode(E2) a discrete Fourier transform (DFT) at the frequency of the waveformof the first electrode (E1) and at its subsequent harmonics, obtainingthe amplitude (M) and phase (Φ) for said frequencies, carrying out saidoperation for each of said experimental conditions corresponding to thepowers (P1, P2, . . . , Pn), and for each of these at the air numbervalues (λ1, λ2, . . . , λm), carrying out for each of said conditions apredetermined number (r) of repetitions, for a total number ofobservations equal to n*m*r, calculating, for each experimentalcondition (i, j), the amplitudes (M1 i,j, M2 i,j, . . . Mpi,j) andphases (Φ1 i,j, Φ2 i,j, . . . , Φpi,j) by applying the discrete Fouriertransform (DFT), where p is the harmonic maximum for which the discreteFourier transform (DFT) is applied, inserting the amplitude (M) andphase (Φ) values into a linear system in which each row is obtained froman experimental observation made at the power Pi and at the air numberλj and in which the known term is λj, setting a number of experimentalobservations (n*m*r) which is greater than the maximum number ofharmonics (p), solving the linear system of the equation AB=λ with Abeing the matrix of experimental data, B the vector of the unknowncoefficients and λ the vector, by the least-squares regression method,of the Moore-Penrose equation whereB=(A ^(T) A)⁻¹ A ^(T) storing in the electronic circuit the coefficientvector B, with a dimension equal to the unknowns of the system or equalto the number of columns of the matrix A, so as to use the followingregression equation:${\lambda_{j} = \left\lbrack {1\mspace{14mu} \left( \frac{M_{2}}{M_{1}} \right)^{s}\mspace{11mu} \left( \frac{M_{3}}{M_{1}} \right)^{s}\mspace{11mu} \left( \frac{M_{4}}{M_{1}} \right)^{s}\mspace{11mu} \left( \frac{M_{5}}{M_{1}} \right)^{s}\mspace{11mu} \ldots \mspace{11mu} \left( \frac{M_{p}}{M_{1}} \right)^{s}\mspace{11mu} {\sin \left( {\phi_{2} - {2r\; \phi_{1}}} \right)}\; {\sin \left( {\phi_{3} - {3\; r\; \phi_{1}}} \right)}{\sin \left( {\phi_{4} - {4\; r\; \phi_{1}}} \right)}{\sin \left( {\phi_{5} - {5\; r\; \phi_{1}}} \right)}\mspace{11mu} \ldots \mspace{11mu} {\sin \left( {\phi_{p} - {p\; r\; \phi_{1}}} \right)}\mspace{11mu} {\cos \left( {\phi_{2} - {2\; r\; \phi_{1}}} \right)}\mspace{14mu} {\cos \left( {\phi_{3} - {3\; r\; \phi_{1}}} \right)}\; {\cos \left( {\phi_{4} - {4\; r\; \phi_{1}}} \right)}{\cos \left( {\phi_{5} - {5\; r\; \phi_{1}}} \right)}\mspace{11mu} \ldots \mspace{11mu} {\cos \left( {\phi_{p} - {p\; r\; \phi_{1}}} \right)}} \right\rbrack}\;$ with s and r which can assume a value in the range [1;4], and p≧5,estimating the air number value, under an actual operating condition, bymeans of the following steps: acquiring the voltage signal at the secondelectrode (E2) for a predetermined time interval, calculating theamplitude (M1, M2, . . . , Mp) and phase (Φ1, Φ2, . . . , Φp) by meansof discrete Fourier transform, calculating the estimated air numbervalue (λstim) by the following scalar product:$\lambda_{stim} = {\left\lbrack {1\mspace{14mu} \left( \frac{M_{2}}{M_{1}} \right)^{s}\mspace{11mu} \left( \frac{M_{3}}{M_{1}} \right)^{s}\mspace{11mu} \left( \frac{M_{4}}{M_{1}} \right)^{s}\mspace{11mu} \left( \frac{M_{5}}{M_{1}} \right)^{s}\mspace{11mu} \ldots \mspace{11mu} \left( \frac{M_{p}}{M_{1}} \right)^{s}\mspace{11mu} {\sin \left( {\phi_{2} - {2r\; \phi_{1}}} \right)}\; {\sin \left( {\phi_{3} - {3\; r\; \phi_{1}}} \right)}{\sin \left( {\phi_{4} - {4\; r\; \phi_{1}}} \right)}{\sin \left( {\phi_{5} - {5\; r\; \phi_{1}}} \right)}\mspace{11mu} \ldots \mspace{11mu} {\sin \left( {\phi_{p} - {p\; r\; \phi_{1}}} \right)}\mspace{11mu} {\cos \left( {\phi_{2} - {2\; r\; \phi_{1}}} \right)}\mspace{11mu} {\cos \left( {\phi_{3} - {3\; r\; \phi_{1}}} \right)}\; {\cos \left( {\phi_{4} - {4\; r\; \phi_{1}}} \right)}{\cos \left( {\phi_{5} - {5\; r\; \phi_{1}}} \right)}\mspace{11mu} \ldots \mspace{11mu} {\cos \left( {\phi_{p} - {p\; r\; \phi_{1}}} \right)}} \right\rbrack \times B}$13. The method according to claim 8 further comprising calculating insaid first phase a plurality of vectors (B) of calibration coefficients,each correlated with respective power bands (P) between the minimum andmaximum admissible power, and at least partly overlapping, in order toachieve greater precision in estimating the air number (λ).
 14. Themethod according to claim 8 further comprising calculating a coefficientvector (B) correlated with the respective gas family for which theburner (1) is intended, to allow said gas family to be identified duringthe burner installation phase.
 15. The method according to claim 1,wherein said burner (1) comprises: a combustion chamber (2), a firstduct (3) capable of introducing air into said combustion chamber (2),first control means (5) associated with said first duct (3), configuredto vary the amount of air introduced into said first duct, a second duct(4) capable of introducing a fuel gas into said combustion chamber (2),second control means (6) associated with said second duct (4),configured to vary the amount of gas introduced into said second duct;said method comprising the phases of: setting one of said first and saidsecond control means (5, 6) to a first setting value, on the basis ofcontrol curves preset in the control circuit, associating acorresponding setting value for the other control means, said valuesbeing correlated with a target air number (λob) that is deemed optimalfor combustion, calculating, under the operating condition achieved, theactual air number value (λstim) by the method of one or more of thepreceding claims, comparing the target air number (λob) with the actualair number (λstim) and correcting one and/or the other of said first andsaid second control means (5, 6) so as to obtain an actual air number(λstim) that substantially coincides with the target air number (λob).16. The method according to claim 15, wherein said first control meanscomprise a fan (5) with a preselected control curve related to a numberof rotations or an air flow rate, and said second control means comprisea gas valve (6) of the modulating type with a preselected control curverelated to a current or a gas flow rate, said setting values being thespeed of the fan (5) and/or the driving current for the modulator of thevalve (6).
 17. A system for controlling combustion in a burner (1) of afuel gas apparatus, operating according to the method of claim
 1. 18.The method according to claim 2, wherein the characteristic parametersof the waveform of the response signals are obtained by applying afunctional transform.
 19. The method according to claim 9, wherein thereis a first sampling frequency of the signal associated with the positivepulses and a second, distinct sampling frequency associated with thenegative pulses.
 20. The method according to claim 12, furthercomprising calculating in said first phase a plurality of vectors (B) ofcalibration coefficients, each correlated with respective power bands(P) between the minimum and maximum admissible power, and at leastpartly overlapping, in order to achieve greater precision in estimatingthe air number (λ).
 21. The method according to claim 12, furthercomprising calculating a coefficient vector (B) correlated with therespective gas family for which the burner (1) is intended, to allowsaid gas family to be identified during the burner installation phase.